Today's internal combustion engines must meet ever stricter emissions and efficiency standards demanded by consumers and government regulatory agencies. Accordingly, automotive manufacturers and suppliers expend great effort and capital in researching and developing technology to improve the operation of the internal combustion engine. Turbochargers are one area of engine development that is of particular interest in this case.
A turbocharger uses exhaust gas energy, which would normally be wasted, to drive a turbine. The turbine is mounted to a shaft that in turn drives a compressor. The turbine converts the heat and kinetic energy of the exhaust into rotational power that drives the compressor. The objective of a turbocharger is to improve the engine's volumetric efficiency by increasing the density of the air entering the engine. The compressor draws in ambient air and compresses it into the intake manifold and ultimately the cylinders. Thus, a greater mass of air enters the cylinders on each intake stroke.
When a turbocharger is sized to provide maximum power output for a particular engine, the turbocharger's low-load and transient response performance is generally less than optimal. A turbocharger's compressor performance is dependent on the compressor speed. In order for the compressor to rotate fast enough to provide significant compression, or boost, to the engine, there must be a corresponding increase in exhaust gas flow. However, there is a time delay while the exhaust gases build up and the inertia of the turbine and compressor wheel assembly is overcome. This time delay between the engine's demand for boost and the actual increase in manifold pressure is often referred to as turbo lag.
To help overcome the problems of turbo lag and low-load performance, electrically-assisted turbochargers have been developed. Electrically-assisted turbochargers include an electric motor that is operative to supplement the rotational power derived from the exhaust during low-load and transient conditions. Typically, the motor is connected to the same shaft that carries the turbine and compressor wheels. In some cases, the motor's rotor magnets are carried directly on the shaft, while the stator is contained within the turbocharger's center housing.
Electric motors are sensitive to heat and contamination. Accordingly, controlling heat and oil migration, which are common issues associated with turbochargers, becomes more problematic in electrically-assisted turbocharger applications. For example, excessive heat may overheat stator coils and may damage permanent magnets. Moreover, oil contamination can create viscous drag between the motor's rotor and stator as well as transport dirt and debris into the gap between the rotor and stator.
Accordingly, there is a need for an electrically-assisted turbocharger design that inhibits oil migration into the motor and provides adequate cooling of the motor components.